If a question is not mandatory, the respondent doesn't have to actively do anything to skip it. That's fine for some questions. But in many cases it is preferable for the respondent to actively indicate that he doesn't have a response (or doesn't know, or refuses to answer) before being allowed to go on. In particular, an active "do not know" option will prevent respondents from skipping questions by accident.
I was hoping I'd see this as a question setting along with "Option 'Other'" and "Mandatory" in 2.0, but alas, not yet a feature.
"Option 'Do Not Know'" should be like "Option 'Other'" in that the survey developer should be able to customize the 'Do Not Know' text.
Alternatively, the "Mandatory" option could be expanded, so that a question is either mandatory (as now), passively optional (as in the current 'not mandatory') or actively optional (in that the user has to specifically select some sort of "skipping this question" radio button.)